1. Field of the Invention
This invention relates to a hollow needle suitable for use in medical applications, such as insertion into a blood vessel or other tissue for hypodermic injection, insertion of a cannula for transfusion, dialysis, intravenous drip, blood-collecting and the like, these functions being generally termed "injections" herein. More particularly, a cannular needle defining a flow path in which a fluid such as a liquid medicine or blood can pass in one direction or the other, is provided with a specifically formed point. The invention also concerns the method for producing the needle and the point involving the formation of an oblique planar bevel, a back bevel and two laterally converging bevels, resulting in a sharp end having a prow shape with three cutting edges converging into an end point.
2. Prior Art
The point of a conventional hollow needle, such as a needle intended for hypodermic injection, is formed by selectively grinding an end of a tube or hollow cylinder to form a point. The tube normally has a narrow diameter and advantageously the point is as sharp as practicable, to facilitate the piercing of tissues.
Typically, the end portion of the cylinder is ground along a flat plane extending in an oblique direction relative to the axis of the tube or hollow cylinder, i.e., in a direction that converges with the longitudinal axis of the cylinder at an acute angle, crossing the axis to the opposite outside wall at the extreme end point. Assuming an arbitrary position of the needle wherein the bevel slopes downwardly from a "top" wall of the cylinder to a point at the outer edge of the "bottom," this planar bevel results in an open bevel end-face on a side of the point wherein the lumen is oval in plan view and is surrounded by an elliptical annulus between the lumen, and the top outer wall and the bottom outer wall (namely the point). Without further grinding, the point forms the rounded chord of the minor side of an ellipse at the extreme end.
If the tapering wall thickness of the extreme end of the cylinder is made very thin during manufacturing, the point is very sharp and will inflict less injury to the skin, blood vessel or other tissue at the point of injection of the patient than will a thicker, duller point. To improve sharpness, the acute angle of the oblique bevel forming the point can be a very small angle, making the point very thinly tapered to the extreme end. Consequently, less pain is felt by the patient during the injection. However, if the acute angle of the oblique bevel is made relatively smaller, the axial extension of the elliptical annulus becomes longer between the lumen or inner wall and the extreme end or point at the outer wall. As a result, there is an elongated point have very little thickness or strength of metal at and adjacent to the extreme end or point of the needle due to the axially elongated and thinly tapered bevel end-face. Needles of this type are disclosed, for example, in U.S. Pat. No. 3,308,822--De Luca and International Application No. PCT/NO91/00104 (International Publication WO 92/04062). With extreme tapering and thinness at the point, the metal approaching the tip becomes as thin as a foil, the tip is difficult to grind in a manner that preserves the shape of a smooth ellipse at the tip, may overheat in grinding and is easily bent.
International Application No. PCT/NO91/0104 also discloses the possibility of a back bevel on a needle having an oblique bevel face as described. Assuming as above that the needle is oriented such that the oblique bevel tapers downwardly from a top wall to the point as above, the back bevel tapers upwardly from the bottom wall to the point. A back bevel is advantageous in that the extreme end or point of the needle is moved radially inwardly from the bottom wall. This reduces the possibility of piercing the far wall of a blood vessel during an intravenous injection. However such a back bevel tends to increase the lateral width of the point of the needle as compared to the shape of the minor chord of an ellipse.
There is a need for an improved needle and method for producing it that will provide an extremely sharp edge portion that is thin, but strong at the same time. Preferably, the improved needle should have the point spaced radially inward from an extension of the outer wall, and preferably the back bevel cut to accomplish this should not unduly thin and weaken the strength of the tip approaching the tip.